Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.
Data is typically written to the disk by modulating a write current in an inductive coil to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During read-back, the magnetic transitions are sensed by a read element (e.g., a magneto-resistive element) and the resulting read signal demodulated by a suitable read channel. A recent development referred generally as energy assisted magnetic recording (EMR) enhances the writing process by augmenting the magnetic field generated by the write coil with an additional energy source. For example, with heat assisted magnetic recording (HAMR) the quality of written data is improved by heating the disk surface during write operations in order to decrease the coercivity of the magnetic medium, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface. In another example, with microwave assisted magnetic recording (MAMR) the quality of written data is improved by using a spin torque oscillator (STO) to apply a high frequency auxiliary magnetic field to the media close to the resonant frequency of the magnetic grains.